OK, YK, too much to say..
1. Try all means of transportation. For instance, take Tunel up from Karakoy to Beyoglu. You get out on Istiklal Caddesi and can walk to Tamsin Square to see the Ataturk monument.

2. Consider a Teras ( high floor roof ) at sunset to see the beautiful view with lights. Pretty great during the day, too.

3. Please know we felt perfectly safe. And were not hassled by restaurant or merchant touts.

4. YMMV nut I found Hagia Sophia a waste of time and money. The first floor is all scaffolded.

5. On the contrary, I found the Blue Mosque wonderful! My suggestion is to be there 8:30 a.m. when it opens.

9. The Basilica Cistern was surprisingly beautiful and fun.

10. Gulhane Park is a lovely place to walk.

11. Topkapi is the better of the two palaces to visit. It does call for several hours.

12. The Hippodrome has very interesting monuments. A guidebook of detail will add to understanding.

Best wishes on all your trips, where are you heading for December break and any other travel before Hong Kong? Your son is very lucky, which I hope he realizes.

from what I read, given our layover is >24hrs, we have to retrieve our checked luggage (they will not be tagged to our destination). I also read there is left luggage facilities at IST airport so we will try to stow them and stick with something light, at most one carryon).

Now that I've checked out the map and read about the sights, it seems like it's easy enough for us to DIY and not needing a guided day tour.

First attempt of an itinerary ( i still have 4 more months to consider)
Saturday 5am arrival

• stow luggage
• find atm, get lira
• buy Istanbulkart (one card for both of us?)

Take the 7am or 7:45am Havaist Bus 11 to Sultanahmet stop. ~ 90 minutes. 7-min walk to hotel. Hotel arrival ~ 9am. drop off luggage, get breakfast.
~ 10am Visit Blue Mosque
Lunch, 1pm hotel check-in, rest, freshen up
Afternoon visit Cistern (pre-book tickets), tram to Spice Bazaar
Late afternoon commuter ferry from Eminonu to Kadikoy and back (time it so returning around dusk, 7:30pm)?
Dinner

Sunday
hotel breakfast (included)
Walk over to Hagia Sophia
If there's time left, check out Grand Bazaar
Back to hotel around 12 to check out, have hotel arrange taxi to IST airport, retrieve left luggage. Hopefully time to relax at the Turkish airline lounge before our flight at 16:40

We will skip Topkapi this time. We just won't have enough time to do it justice, and knowing that we will be tired on our arrival day anyway. Can save it for future visit!

thank you NYCMOM50! We are staying put for December break, as we normally do. Too expensive and too hectic to travel during that that time!

my son gets 1 week off for Feb break and we are going to southern Spain (Malaga) with my sister-in-law ( husband's sister). My husband won't be joining as he has limited vacation time and he needs to watch his sister's cat while she's with us in Spain 😆

I also have a couple of solo short NYC jaunts in the new year to catch some concerts and operas ☺️

Okay much detailed, anyways I would suggest a trip to Bosphorus, you will enjoy the viewary there, also a lots of local shop for you to explore there.

I must admit a bit of surprise at this post. For one thing, Bosphorus is not a place -- it is a waterway. Where, exactly, muntiqa thinks there are local shops is a mystery to me.

Several of us have already said that we would make a boat ride along the Bosphorus a low priority. I think an advantage of Fodor's forums is that people with differing views can share them .. but timeliness would seem important. And saying WHY it might be worthwhile, when so many argue against it, would seem useful. IMO, a ride on the Bosphorus can offer views. So can other places in Istanbul. IMO, other views are beter. I stand by my recommendation to avoid it.

Your plan sounds just righT!

i would say one IstanbulKart for two, I did that, too.

The Grand Bazaar is worth seeing one aisle and you don’t need much time there. IMO

The Spice Market is more interesting, to me, anyway. And it won’t take long.

The Blue Mosque was one of my favorites and free to enter. Very large, too.i would just check if it is closed at prayer time meaning you cannot enter. I do not recall. And prayer times vary with time of year.

The skyline is thrilling and the call to prayer is moving.

As you know, many ATMs there charge a big fee: 9.9%.

The Lounge at IST is a big cut above and big! Try to arrive hungry. There are more food stations than a food court. Even a barista. There is a person who will press clothes! There are clear storage lockers if you want to stow your carry-on while there. Honestly, among the best I have seen on 4 continents.

Enjoy and thanks for mentioning your other travel plans.